A premixed compression ignition type engine that is operated while changing over between spark ignition (sometimes referred to as SI) combustion and compression ignition (sometimes referred to as HCCI or homogeneous charge compression ignition) combustion is controlled, for example, under the control method disclosed in Japanese Patent Application Laid Open No. 2000-220458, or No. 2004-293458.
In JP2000-220458, the ignition timing of the engine is retarded to make compression ignition precede spark ignition in making a changing over from spark ignition combustion to compression ignition combustion, so the transition to compression ignition combustion is made reliably and smoothly. As the ignition timing is thus retarded gradually, the pressure generated during combustion, namely, output such as torque decreases gradually. Therefore, the amplitude of fluctuation in torque (torque step) can be held small, and the change over to compression ignition combustion can be made with a low output. Accordingly, this control method makes it possible to cause a drop in in-cylinder temperature and suppress combustion noise and the like resulting from pre ignition or knocking during a change over period from spark ignition combustion to compression ignition combustion.